All of the above conclusions indicate that a mortar may be quite porous, and yet so long as the voids are very minute, the percolation of water through it will be slow. This is especially shown by the fact that mortars of coarse sand, not porous, are more permeable than the porous mortars of fine sand.
485. When water is permitted to percolate continuously through a mass of mortar, the interstices gradually become filled, and the permeability decreases in marked degree. M. Alexandre found that a volume of water which passed a certain mass of mortar in twenty minutes at the beginning of the ex periment, required five hours to percolate the mass at the end of a month. M. R. Feret has obtained similar results in making extensive experiments1 on the subject of permeability, and considers that fine particles of cement or lime are carried along by the water, forming efflorescence at the surface and tending to stop the flow.
To enumerate briefly the precautions necessary to attain water-tightness in mortars and concretes, it may be said that different brands of cement present different characteristics in this regard. Fine grinding is a prime requisite, and sand cement or silica cement, containing as it does very fine grains of sand intimately mixed with cement particles of extreme fineness, is admirably adapted to such uses.
The sand should, if possible, be composed of a mixture of grains of various sizes, because such a mixture gives a mortar not only little permeable, but one that is not porous, and that has, besides, a good strength. The amount of cement in the mortar should be in excess of the voids in the sand, not less, in general, than three barrels of cement per cubic yard of sand.
In concrete the volume of mortar should exceed the volume of voids in the aggregate, and to obtain this result without too great expense, the aggregate should be so selected as to have a minimum of voids. Gravel concrete properly proportioned may be made water-tight somewhat more easily than broken-stone concrete, but a mixture of gravel and broken stone will give good results not only in this regard, but in the matter of strength as well.
487. To make a compact mortar for use where the facilities for tamping are ordinarily good, the consistency should be neither very wet nor very dry. When the mortar is struck with the back of a shovel, moisture should glisten on the surface, but in a pile the mortar should appear but little moister than fresh earth. This is the consistency which, with a moderate amount of tamping, gives the least volume of mortar with given quantities of dry materials. In places difficult of access, or in the preparation of concrete, better results will be obtained with a mortar somewhat wetter than the above, since large voids will be less likely to occur in the more plastic mass. In fact, unless the supervision is very close, it is advisable to use a rather wet mixture in preparing concrete where water-tightness is desired.
1 "La Capacity des Mortier Hydrauliques," Annates des Ponts et Chaussees, July. 1892.
The application of certain washes to the surfaces of walls intended to be water-proof, and the introduction of foreign materials into the mortar or concrete to make it less permeable, have been practiced to some extent. Alternate coatings of soap and alum solutions are applied with a brush, not only to concrete, but to brick and stone masonry surfaces. These penetrate the pores of the masonry, forming insoluble compounds which prevent percolation. Washes of grout, composed of cement, or of cement and slaked lime, are used for a similar purpose.
"Sylvester's Process for Repelling Moisture from External Walls" consists in applying first a solution of three quarters of a pound of soap to one gallon of water, followed, after twenty-four hours, by the application of a solution containing two ounces of alum per gallon of water. Both solutions are applied with a brush, the soap solution boiling hot, and the alum solution at 60° to 70° Fahr. The applications are alternated, with twenty-four hours intervening each time. Experiments at the Croton Reservoir 1 indicated that four coats of each wash were required to render brickwork impervious to a head of forty feet of water and the Cost of the four double applications was about ten cents a square foot.
In Reservoir Number Two of the Pennsylvania water Co., two washes of each solution were used on the walls at a Cost for materials and labor of twenty-three cents per hundred square feet, and the results were said to be good.
A modified recipe for such a wash in which but one solution is made is given as follows:2 A stock solution is prepared of one pound lye, five pounds powdered alum, dissolved in two quarts water. One pint of the stock is used to a pail of water in which ten pounds Portland cement has been well mixed.
1 Trans. A. S. C. E., Vol. i, p. 203.
2 J. H. G. Wolf, Engineering News, June 30, 1904.
489. In a few cases the use of alum and soap solutions in the body of the mortar has been tried with apparently successful results. Mr. Edward Cunningham,1 in making experiments on water-proof concrete vessels, used powdered alum equal to one per cent, of the combined weight of the sand and cement, mixing this with the dry ingredients. To the water used in. mixing, one per cent, of yellow soap was added. The results were said to be very satisfactory. In the above proportions, however, the amount of alum is made to depend upon the amount of cement and sand used, while the soap added depends upon the amount of water, whereas the soap should bear a definite ratio to the alum.
In experiments with mortar composed of one part cement to two and one-half parts of bituminous ash, Prof. W. K. Hatt 2 found that the alum and soap mixed with the mortar at the time of gaging increased the strength and hardness of the ash mortar about fifty per cent., and diminished the absorption by the same percentage. One half of the water used for gaging was a five per cent, solution of ground alum, the other half being a seven per cent, solution of soap. The alum solution was used first and the gaging completed with the soap solution.